SQL Sum with Conditions in Two Columns

As a beginner in SQL and VB.NET, you’re likely looking for ways to manipulate data from an existing table called STOCK. In this article, we’ll explore how to calculate the current quantity by summing up the buy and sell values.

Understanding the Current Quantity Calculation

The goal is to display the current quantity amount in each row, which can be achieved by adding the current buy value and subtracting the current sell value. This calculation is equivalent to:

Current Quantity = (Current Buy - Current Sell)

To achieve this, we’ll need to use SQL’s window functions, specifically SUM() with an OVER clause.

Table Structure

Let’s take a closer look at the existing table structure:

+----+------------+--------+-------+---------------+
| ID | Date       | BUY    | SELL  | Current Quantity |
+----+------------+--------+-------+---------------+
| 1  | 01/01/22   | 88     | 0     |                |
| 2  | 03/01/22   | 22     | 0     |                |
| 94669|05/02/22    | 0      | 30    |                |
+----+------------+--------+-------+---------------+

Notice that the Current Quantity column is blank for each row, as we want to calculate it dynamically.

SQL Query

The provided answer uses a query with two parts:

1. A subquery to calculate the net sell value (buy - sell) for each row.
2. The outer query to sum up the net sell values over time using the OVER clause.

Let’s break down this query:

SELECT 
    a.*,
    SUM(net_sell) OVER (ORDER BY Curr_date) AS Current_quantity
FROM (
    SELECT s.*, 
           buy - sell AS net_sell
    FROM STOCK s
) a;

Here’s what happens in the subquery:

• We select all columns (*) from the STOCK table.
• We calculate the net sell value for each row by subtracting sell from buy.
• The results are wrapped in an alias a.

The outer query uses the SUM() function with an OVER clause to sum up the net sell values over time. We specify:

• ORDER BY Curr_date: Order the rows by date, which allows us to calculate the cumulative sum.
• AS Current_quantity: Alias the resulting sum as Current_quantity.

Explanation

The key to this query is the use of a window function (SUM() with OVER) to calculate the cumulative sum over time. By ordering the rows by date and specifying the column to sum up, we can effectively calculate the current quantity for each row.

Here’s an example walkthrough:

• Row 1 (01/01/22): Net sell = 88 - 0 = 88. Current quantity = 88.
• Row 2 (03/01/22): Net sell = 22 - 0 = 22. Since we already have a cumulative sum of 88 from the previous row, we add 22 to get a new cumulative sum of 110. Current quantity = 110.
• Row 3 (05/02/22): Net sell = 0 - 30 = -30. We subtract this from our cumulative sum of 110 to get a new cumulative sum of 80. Current quantity = 80.

And so on.

Conclusion

In conclusion, calculating the current quantity by summing up buy and sell values in two columns requires the use of SQL’s window functions. By using SUM() with an OVER clause, we can effectively calculate the cumulative sum over time, giving us the desired output.

We’ve also explored how to structure a query that performs this calculation, including:

• Using a subquery to calculate net sell values for each row.
• Wrapping the subquery in an outer query to perform the window function calculation.
• Aliasing the resulting sum as Current_quantity.

With this knowledge, you should be able to adapt this approach to your own SQL queries and VB.NET projects.

Last modified on 2024-05-08